Semiconductor lasers are widely used as a light source for ultra-high speed and long-distance optical communications. However, in conventional semiconductor lasers, there is a problem that, due to a very little light reflecting back from an optical fiber, noises occur in the semiconductor laser. Therefore, an optical isolator is indispensable for removing the light reflected back into the semiconductor laser. In general, the optical isolator is composed of a magneto-optic crystal with the Faraday effect and two optical polarizers which sandwich the magneto-optic crystal therebetween and whose polarization directions in transmission are shifted by 45.degree.. In addition, such an optical isolator is composed of at least four components including a magnet.
However, since the conventional optical isolator has the complicated composition and requires many steps for processing materials and fabricating those components, it is costly and may be short of reliability or stability. Furthermore, it is impossible for the conventional optical isolator to be monolithically integrated with the semiconductor laser.